The present invention relates to an ink drop ejection method and an ink drop ejection device for an inkjet printer.
Conventionally, inkjet printers have been well known and wide spread. Japanese Patent Publication No. 3288482 discloses an example of a color inkjet printer. According to this publication, the color inkjet printer has a plurality of multi-recording heads, each being provided with a plurality of recording elements (i.e., nozzles). A single power-supply belt (a flexible flat cable) is provided to supply the power to all the recording heads. In order to reduce the number of power lines embedded in a belt member to reduce load to the movement of a carriage and to the power-supply lines due to the movement of the carriage, driving voltage pulses supplied to the driven elements (nozzle heads) of, for example, cyan, magenta and yellow inks are shifted by one clock period.
U.S. Pat. No. 6,575,565 B1 discloses an on-demand inkjet printer, teachings of which are incorporated herein by reference. In the U.S. Patent, an array of a plurality of orifices (nozzles) are formed on an orifice (nozzle) plate and an array of a plurality of ink channels (pressure chambers) corresponding thereto are provided. Each of the pressure chambers are supplied with ink. On a back surface of the orifice plate, a piezoelectric actuator is provided. The piezoelectric actuator is configured such that a common electrode and individual electrodes are alternately laminated with a piezoelectric ceramics plate (i.e., a piezoelectric sheet) being sandwiched therebetween. Active portions, which are portions between opposing individual electrodes and common electrode in the laminated direction overlap, viewed from the top, above the ink channels (i.e., the pressure chambers) is provided. According to this structure, as a driving voltage is applied to each active portion of the piezoelectric actuator, the active portions deform and decrease capacity of corresponding ink channels (i.e., pressure chambers). Then, the ink inside the ink channel (pressure chamber) are ejected from the orifices (nozzles), thereby an image is printed on an object.
In the above structure, when the pressure chambers are arrayed, barrier walls are provided between adjoining pressure chambers. However, when a driving voltage is applied to an active portion of the piezoelectric actuator, deformation of the active portion exerts an influence on the adjoining pressure chamber in some degree. That is, when a plurality of nozzles are formed on the same member, mechanical vibration due to actuation of one nozzle propagates and affects another nozzle. Therefore, when an ink drop is ejected from a certain nozzle, ink drops may be ejected from the adjoining nozzles simultaneously, or ink ejection speed and/or ejection amount may be changed. Such a phenomenon in which the ink ejection conditions of nozzles interfere with each other is called crosstalk, and has been known as a problem in this field.
It should be noted that if the density of the nozzles is higher, the thickness of the barrier walls becomes thinner and thus the crosstalk occurs easily. Further, for color recording, a plurality of arrays of nozzles are arranged in one print head, and further, the clearance between the adjoining arrays is made small for downsizing, the thickness of the wall between the adjoining arrays is also decreased. Thus, the crosstalk may easily occur in such a construction.
As above, because of the need of the high density of the nozzles and downsizing of the recording heads, both the crosstalk due to the close arrangement of the pressure chambers in the same array and the crosstalk due to the close arrangement of the nozzle arrays occur.
To avoid the crosstalk, rigidity of a member surrounding the pressure chambers may be increased and/or the structure of the piezoelectric actuator may be changed as in the above-described U.S. Patent. However, if the hardware configuration is changed to increase the rigidity, manufacturing/assembling costs increase easily.
FIG. 4 of the aforementioned Japanese Patent No. 3288482 shows ejection pulses which are applied to driving terminals of respective nozzles with a certain time-lag therebetween (i.e., the ejection pulses are delayed). In FIG. 4, the width of the pulses are the same. Such a configuration implies that the amount of ink drops ejected from respective nozzles are the same.
Practically, to express gradation with the inkjet printer, the amount of the ink ejected from a nozzle is varied by changing the width of the driving pulse. By changing the width of the driving pulse, a drop of ink containing a relatively small amount of ink (which will be referred to as a small drop, hereinafter) or a drop of ink containing a relatively large amount of ink (which will be referred to a large drop, hereinafter) can be ejected. When the small drop of ink or large drop of ink is ejected, it is also necessary to impose a delay between the pulses applied to the nozzles respectively ejecting the large drop of ink and small drop of ink.
When the small drop of ink is to be ejected, a feeble pressure is applied to the pressure chamber to eject the drop of ink. If the adjoining nozzle is driven to eject a large drop of ink at the same time, a crosstalk occurs due to large energy for ejecting the large drop of ink, which crosstalk exerts an influence on the nozzle which is to eject the small drop of ink. In such a case, the nozzle which is to eject the small drop may not eject the small drop of ink having an accurate amount, or the ejection speed of the small drop may vary. Such an influence of the crosstalk is significant particularly among the pressure chambers in the same arrays.